1. Field of the Invention
This invention relates to annealing furnaces and methods of annealing and, in particular, to an efficient furnace that can anneal steel laminations without inducing distortions therein and to a method for quickly and efficiently annealing steel.
2. Prior Art
In annealing, a material is heated to a predetermined elevated temperature, temporarily held at that temperature, and then cooled to room temperature. This heat treatment changes some of the physical properties of the material; most notably, hardness is reduced.
Although glass may be annealed, this invention is primarily concerned with annealing metals, and particularly concerned with annealing steel.
It is frequently desirable to anneal steel laminations for use in electric motors and electric transformers. Typically low carbon silicon steel is used for these applications. If the steel laminations are eventually going to be used in transformers, it is necessary that the steel have a quality known as oriented grain. On the other hand if the end use of the steel lamination will be in an electric motor, non-oriented grain steel is required. In either case, it is extremely important that the sides of the laminations be planar after annealing. This is necessary to insure uniform and predictable magnetic fields and their effects about and upon the transformers and electric motors.
At present, the method of annealing steel entails the insertion of the material to be annealed (commonly called a charge) into a furnace operating at a high temperature. The charge is placed into the furnace at room temperature and remains in the furnace until its temperature has been raised to that of the furnace. After a predetermined time, the charge is removed from the furnace and allowed to cool.
Strain fields may be present in the charge to be annealed. These fields, which extend over long distances on an atomic scale, represent a certain amount of strain energy of the charge. During annealing these fields are rearranged into configurations of lower strain energy, with the rearrangement frequently causing distortions in the shape of the charge. These distortions may adversely affect the planarity of the steel laminations rendering them unsuitable for electrical uses.
One of the difficulties with present annealing furnaces is that they can not prevent the loss in planarity that frequently accompanies annealing steel laminations.
Another disadvantage of prior art annealing furnaces is that they require a large amount of energy to anneal each charge. These furnaces are particularly inefficient in that the heat released by the cooling charge is wasted. In addition, each charge successively is heated from room temperature which requires considerable time and energy.
The time required to anneal each charge presents another disadvantage in prior art furnaces. Since each charge must be heated and cooled before the next charge enters the furnace the annealing cycle is correspondingly longer than if the charges were cooled outside the furnace.